1. Field of the Invention
The present invention is directed to a method and apparatus for matching the print outputs of two printers which have different printer characteristics so that the output images of the printers are visually close in appearance.
2. Background Description
Most printers today can print in only a limited number of colors. Digital halftoning is a technique for printing a picture (or more generally displaying it on some two-dimensional medium) using small dots with a limited number of colors such that it appears to consist of many colors when viewed from a proper distance. For example, a picture of black and white dots can appear to contain grey levels when viewed from some distance. For purposes of simplicity, the rest of the discussion will be restricted to the case of grayscale images and their rendering by black and white printers to facilitate the presentation; however, anyone versed in the art of digital halftoning should know how to adapt the present invention to color images. When we speak of ink, it could mean any material and/or mechanism which produces the black in the image, i.e., it could be toner for a xerographic printer, ink for an inkjet printer, etc.
We will be concerned with bilevel, fixed pixel size printers (for instance laser printers). Such printers have two fundamental characteristics:
1) The print resolution, say d dpi (dots per inch), which can be interpreted as saying that the intended fundamental units of the print are arranged on a grid of squares with each square or pixel of size 1/d inches by 1/d inches, where d typically varies from about 300 to about 3000. In some cases, the pixels lie on a rectangular grid, but the discussion adapts equally well to this case, so we will always assume square pixels for definiteness.
2) The dot gain which tells us how the actual printed pixel (or dot) differs from a perfect 1/d by 1/d square in shape and size (notice that in previous sentences, the word "dot" was used in a loose sense). While many printers perform differently, standard theory and much of the prior art on calibration assumes that printers print dots which can be reasonably described as round, say with diameter D (or as an ellipse in the case of a rectangular grid), and the dot gain is often described accordingly.
In the sequel, we make the assumption that no printed dot goes beyond a circle with diameter 2/d centered at the middle of the pixel where it is intended to be printed (a circular dot which covers an entire 1/d by 1/d square has diameter at least .sqroot.2/d ). This assumption is made to simplify the discussion and in particular the description of the invention. Adaptation to a more general case is tedious to describe but not difficult to implement by anyone skilled in the art of digital printing.
While standard theory assumes that printed dots are reasonably described as round, say with diameter D, many printers perform differently. As taught in the invention disclosed in U.S. patent application Ser. No. 09/085,094 filed on May 26, 1998, now U.S. Pat. No. 5,943,477, issued Aug. 24, 1999, by A. R. Rao, Gerhard R. Thompson, Charles P. Tresser and Chai Wah Wu for "Microlocal Calibration of Digital Printers", both the probabilistic nature of individual dot printing and the way printing neighboring dots in various configurations affects the dot shapes can be captured by a calibration method. This method characterizes a printer by the probability distribution of what area of ink gets printed at each pixel depending on the configuration of dots to be printed in the neighborhood of that pixel. Here we use the word "ink" as a generic name for what gets printed, such as ink or toner. In the sequel, whenever we speak of printer characteristic, we mean the characteristic as given by the calibration method as disclosed in U.S. patent application Ser. No. 09/085,094, now U.S. Pat. No. 5,943,477, issued Aug. 24, 1999, except when otherwise specified. The probabilistic nature of such printer characterization means in particular that the notation "fixed pixel size" for a printer refers to its idealized properties rather than to actual ones.
Consider now some grayscale image to be printed with a digital printer. We assume that the image is of size h by .nu., where h and .nu. are expressed in inches to be consistent with the unit used in the dpi description. It is then convenient to interpret this image as a matrix I of size H=h.times.d by V=.nu..times.d in the following way:
One thinks of the image as covered by little squares of size 1/d by 1/d (also called pixels). PA1 Each pixel p, can be designated by its horizontal ordering number i (say from left to right) and its vertical ordering number j (say from top to bottom). Thus, the location of p is specified by the pair (i,j). PA1 To the pixel at (i,j) one assigns the value g between 0 and 1, where 0 corresponds to white, 1 corresponds to black, and more generally, g corresponds to the grey level of this particular pixel. PA1 The matrix I is then defined by setting I(i,j)=g.
Given a matrix such as I, a digital halftoning algorithm will associate to it an H by V halftone matrix M whose entries M(i,j) are either 0 or 1. Now 0 means that no dot will be printed by the digital printer at pixel (i,j), while a 1 means that a dot is to be printed.
A grayscale image can thus be thought of as an array I of B-bit numbers, where typically B ranges from 4 to 12. Because M is an array of single bits of the same size as I, straightforward storage of M instead of I represents a factor B in compression. It often happens that the original grayscale image I is not available and only the halftone (or printing decision) matrix M is retained. Usually, because different printers have different characteristics, a given M will produce different images when used with two different printers. This invention will disclose a method to overcome this problem.